As shown in FIG. 1, a conventional buck-boost power converter 10 includes a power stage 12 to convert an input voltage Vin to an output voltage Vout according to control signals PWM1 and PWM2 for a load RL, voltage divider resistors R1 and R2 to divide the output voltage Vout to generate a feedback signal VFB, an error amplifier 20 to amplify the difference between the feedback signal VFB and a reference voltage Vref to generate an error signal VEA, a compensator 18 to compensate the error signal VEA for stabilization enhancement of the entire system, a ramp generator 22 to provide ramp signals SAWbuck and SAWboost, and comparators 14 and 16 to generate the control signals PWM1 and PWM2 according to the error signal VEA and the ramp signals SAWbuck and SAWboost. FIG. 2 is a waveform diagram of the buck-boost power converter 10 operating in a buck mode, in which waveform 24 represents the input voltage Vin, waveform 26 represents the ramp signal SAWbuck, waveforms 28 and 30 represent the error signal VEA, and waveform 32 represents the control signal PWM1. The buck-boost power converter 10 uses a negative feedback control loop to modify the impact caused by the variation of the input voltage Vin and the variation of the load RL on the output voltage Vout. Taking the variation of the input voltage Vin for example, when the input voltage Vin drops down from Vin1 to Vin2, as shown by the waveform 24 at time t 1, due to insufficient energy the output voltage Vout will decrease first before the feedback loop responds thereto. Since the feedback signal VFB is connected to a negative input of the error amplifier 20, the error signal VEA increases from VEA1 to VEA2, as shown by the waveforms 28 and 30. In addition, since the buck ramp signal SAWbuck has a fixed amplitude and a fixed period, as shown by the waveform 26, the duty of the control signal PWM 1 will be increased, as shown by the waveform 32, to pull the output voltage Vout back to the preset value. However, the error amplifier 20 needs compensation for stability of the power converter 10. Therefore, the closed loop and the compensator 18 will make the error signal VEA being unable to have fast line response, i.e. instant response to the variation of the input voltage Vin, and in consequence the output voltage Vout could not be rapidly recovered to the preset value.
Therefore, it is desired a solution for a buck-boost power converter to achieve fast line response to stabilize the output voltage.